A major distinguishing characteristic of behavioral assessment is the direct assessment of overt behavior. Direct assessment is assumed to provide a sample of behavior that reflects client performance in the situation in which behavior is assessed, even if the assessment procedures were not implemented. Yet, in the majority of investigations, behavioral assessment procedures are obtrusive, i.e., subjects are aware that their behavior is being assessed. The potential problem with obtrusive assessment is that it may be reactive, i.e., affect how subjects perform. Recent research has demonstrated that obtrusive observations often are reactive and that behaviors assessed under obtrusive and unobtrusive conditions bear little relation. From methodological and applied perspectives, additional attention needs to be given to unobtrusive measures of behavior change. The present paper illustrates unobtrusive measures in behavior modification including direct observations, archival records, and physical traces of performance. In addition, validation and assessment problems, questions about the obtrusiveness of the measures, and ethical issues are discussed.

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